A typical feedback control system includes a plant and a controller. The plant may be a machine or other device, the operation of which is desired to be controlled. The controller receives an input reference or command signal and one or more feedback signals representative of one or more outputs of the plant. The controller, via a suitable control law (or filter or compensator or control), processes these signals and supplies one or more signals to the plant so that the plant operates consistent with the input command signal. Feedback control systems, such as this, have been, and continue to be, implemented in numerous and varied environments to control various types of machines or processes. For example, many systems include actuators that are used to controllably position one or more loads. These actuators are, in many instances, controlled using a servo control system.
Depending on the particular load that is being controlled, a servo control system may be subject to bi-directional loads and relatively high inertia. In these instances, the servo position control system will typically include a speed loop. The speed loop, if included, controls overshoot of speed or position, improves accuracy, and enhances stability. To implement the speed loop, speed is either measured directly or is derived from measured position, to supply a speed feedback signal. In both instances, precision sensing devices are used to measure speed or position. For example, a suitably precise tachometer may be used to measure speed, and a suitably precise resolver may be used to measure position. As may be appreciated, these speed or position sensing devices may add size, weight, complexity, and concomitant costs to the control loop. Hence, if the speed feedback signal could be eliminated from the speed loop, then the tachometer and/or the resolver could also be eliminated in favor of relatively less complex position sensors. For example, if the servo position control system includes a brushless DC motor, then relatively simple discrete Hall effect sensors, which may be used for motor commutation, may instead be used.
Without speed feedback, a position-only servo control system may suffer an undesirable amount of droop (position error) in the presence of high loads. As is generally known, this droop can be overcome by including an integrator in the position loop, thereby implementing PI (proportional-plus-integral) control. Unfortunately, the integrator may aggravate position overshoot, especially in the presence of aiding loads. As is also generally known, a lead-lag filter in the position loop may be included to combat overshoot and enhance stability. Moreover, if an integrator is included in the position loop, as described above, a lead-lag filter may also offset, at least partially, the above-noted drawbacks of the integrator. Unfortunately, the lead-lag filter may exhibit drawbacks of its own. For example, it may exhibit sensitivity to command step size, whereby a relatively small step command may cause undesirable overshoot and a relatively large step may command may cause excessive undershoot.
Hence, there is a need for a servo control system that does not rely on relatively complex and costly sensing devices and/or does not exhibit an undesirable amount of droop in the presence of high loads and/or does not exhibit undesirable position overshoot and/or does not exhibit sensitivity to command step size. The present invention addresses one or more of these needs.